To synthesize aromatic primary amino compounds a desirable method is to hydrogenate corresponding aromatic nitro compounds in the presence of a hydrogenation catalyst. For some of these reactions a wide selection of catalysts is available. A problem arises, however, if the precursor composition, i.e., the aromatic nitro compound, contains sulfur. Even a small amount of sulfur as a contaminant or as a component of the precursor will quickly inactivate the catalyst or, as commonly stated, will poison it.
To remove sulfur that may be present in the precursor composition is expensive and often ineffective since even small amounts of unremoved sulfur will still poison the catalyst. This has been an obstacle to the economical synthesis of certain aromatic amino compounds which are used for making dye-forming couplers for color photography inasmuch as their precursor nitro compounds are made from sulfur-containing compounds. Although the pure nitro compounds themselves contain no sulfur, the reaction product in which they are obtained, unavoidably contains substantial amounts of sulfur which is extruded from sulfur-containing precursors. Even with careful recrystallization, the purified nitro compound may contain from 500 to 50,000 parts by weight per million (ppm) of sulfur, calculated as elemental sulfur. The sulfur may be present as elemental sulfur or as sulfur compounds. In either event the sulfur will poison the types of catalysts which have heretofore been considered to be sufficiently active and selective for hydrogenating nitro-substituted aromatic compounds to corresponding primary amines.